You are here

Hypoxia

In the summer of 2002, oxygen levels in the water near the Oregon coast plunged so low that fishes, crabs, and other marine organisms had to flee or die in the suffocating waters. The low oxygen levels, commonly called hypoxia, observed during 2002 were more extreme than had ever been documented so shallow and close to the shore off Oregon during the prior 50 years of oceanographic cruises. However, since 2002 coastal hypoxia has recurred off of Oregon every summer, including many years with severe hypoxia.

PISCO scientists are working to understand how upwelling, a normal event that brings rich, life-giving water to the Oregon coast, can turn deadly, causing one of the largest known hypoxic "dead zones" in the world. PISCO scientists are also looking closely at how Oregon’s marine life is responding.

Since 2002, hypoxia has occurred every summer, although the size, duration, and severity of the dead zone varies from year to year. The map below shows the known extent of hypoxia in 2006 and 2007. The most severe event occurred in the summer of 2006 when oxygen levels dropped to new historic lows including some measurements with no oxygen (anoxia), and hypoxic water could be found in large areas along the Washington and Oregon coasts.

How does the Pacific Northwest hypoxia form?

Every summer off Oregon and Washington, northerly winds and the earth’s rotation combine to drive surface waters offshore and bring naturally nutrient-rich but oxygen-poor waters to the coast. When this water wells up to the ocean surface, microscopic plants called ‘phytoplankton’ bloom in the sunlight creating a rich broth that feeds a variety of fish and marine invertebrates. It is this upwelling of nutrient-rich waters that makes the west coast of North America one of the most productive marine ecosystems on earth.

But upwelling is a double-edged sword. The rich broth of phytoplankton that blooms at the surface ultimately sinks toward the sea floor where it decomposes. The process of decomposition consumes oxygen from the water column, and far from the surface, this oxygen is not easily replaced. The upwelling process can thus lead to oxygen depletion near the sea floor and extending up through two-thirds of the water column. Hypoxia does not usually affect the sea surface or areas close to shore, where breaking waves efficiently mix oxygen from the atmosphere into the water.

If coastal winds become southerly (downwelling-favorable), surface waters to move back toward the shore, driving bottom waters away from the coast -- a process called downwelling. When upwelling periods alternate with periods of sufficiently strong downwelling, low-oxygen waters do not accumulate near the seafloor. During seasons with strong upwelling not punctuated by downwelling, low-oxygen waters can accumulate, causing a dead zone. Each successive pulse of upwelling brings the low-oxygen waters closer to shore and more nutrients to the lighted zone. The resultant phytoplankton blooms eventually sink and decay, further depleting oxygen near the sea floor.

Repetitions of these events cause the mass of low-oxygen water near the sea floor to gradually become thicker as well as lower in oxygen. Changes in the strength and pattern of upwelling winds and the oxygen and nutrient content of the deep offshore waters thus impacts the likelihood and severity of hypoxia events. The normal upwelling period runs from about April to September. October to March is generally dominated by downwelling-favorable coastal winds. Even during years in which hypoxia develops in the summertime, high-oxygen conditions have reappeared in October, with the beginning of downwelling that pushes low-oxygen waters away from the coast.

The New Normal

It is normal to find naturally low-oxygen conditions in deep, offshore waters, e.g., at the edge of the continental shelf and slope. However, the occurrence of low-oxygen water close to shore (the inner shelf, less than 50 m (165’) of water) is highly unusual and had not been reported prior to 2002 despite over 50 years of scientific observations along the Oregon coast. Moreover, the appearance of anoxia, or zero-oxygen conditions, in 2006 was unprecedented and resulted in mass die-offs of long-lived marine animals such as seastars and sea cucumbers. The presence of long lived species on the seafloor, with some individuals likely decades old, suggests that such low-oxygen conditions are not normal in this system.